The present invention relates generally to two stroke diesel engines, and more particularly to two stroke homogenous charge compression ignition engines that utilize a pulsed air supply.
Engineers are constantly seeking new ways to improve the efficiency and reduce emissions from diesel engines. One recently recognized avenue for reducing emissions is commonly referred to as homogenous charge compression ignition. Unlike a conventional diesel engine, a homogenous charge compression ignition engine injects fuel into the combustion space when the engine piston is closer to its bottom dead center position than it is to its top dead center position. By injecting fuel early in the compression stroke, the fuel has a better opportunity to mix with the air in the cylinder such that a relatively homogenous lean mixture of air and fuel is present when ignition occurs at a time when the engine piston is near its top dead center position. Although the strategy of utilizing homogenous charge compression ignition can result in substantial reductions in emissions, such a strategy only appears to work well at engine loads less than about 50%. Thus, a strategy in a four stroke diesel engine that utilizes homogenous charge compression ignition at lower engine loads appears promising; however, utilizing such a strategy at higher engine loads remains problematic.
Although two stroke engines have gained a reputation of producing high emissions over their four stroke counterparts, engineers have began to revisit modified two stroke engines as a possible strategy for an improved diesel engine. For instance, SAE Paper No. 790501 entitled Active Thermo-Atmosphere Combustion (ATAC)xe2x80x94A New Combustion Process For Internal Combustion Engines describes a two stroke engine operation that utilizes homogenous charge compression ignition in a partly loaded engine. However, this reference fails to suggest an efficient scavenging strategy, and is thus plagued by some of the same problems that have historically been linked to two stroke engines. This reference also fails to provide any suggestion as to how to improve efficiency across an engine""s speed and load range.
In another example two stroke engine, such as that described in U.S. Pat. No. 5,265,564 to Dullaway, scavenging is made substantially more efficient by the utilization of a pumped air supply that permits the usage of conventional poppet type intake and exhaust valves. The combination of a pulsed air supply with conventional poppet type intake and exhaust valves appears to permit an efficient scavenging process by allowing the valves to open and close when pressure differentials across the valve are relatively low, which appears to be a gain not easily realizable in a conventional turbocharged engine. Although Dullaway appears to teach some improvements, especially in the area of gas exchange in a two stroke engine, it continues to rely upon a conventional diesel injection strategy that results in higher emissions. In other words, Dullaway continues to promote fuel injection when the engine piston is near its top dead center position which results in undesirably higher emissions. In addition, Dullaway also appears to suffer from an inability to improve performance across the engine""s operating range, by having event timings apparently optimized for a single engine speed.
The present invention is directed to these and other problems associated with diesel engine technology.
In one aspect, a two stroke homogenous charge compression ignition engine includes a reciprocating piston positioned in a cylinder defined by an engine housing. A fuel injector is attached to the engine housing and is at least partially positioned in the cylinder. The fuel injector includes a nozzle outlet that begins to open at least once during each reciprocation of the piston when the piston is closer to a bottom dead center position than to a top dead center position. An intake valve is positioned above the piston and opens at least once during each reciprocation of the piston. Likewise, an exhaust valve is positioned above the piston and is opened at least once during each piston reciprocation. A volume pulsed air supplier, which produces at least one volume pulse with each piston reciprocation, is fluidly connected to the cylinder when the intake valve is in its open position.
In another aspect, a method of operating an engine includes a step of scavenging a cylinder at least in part by opening an intake passage and an exhaust passage to the cylinder. A volume pulse of air is sent to the cylinder via the intake passage during each reciprocation of the power piston in the cylinder. Fuel is injected at least once during each reciprocation of the power piston into the cylinder when the power piston is closer to a bottom dead center position than to a top dead center position.